Registration Dossier

Diss Factsheets

Environmental fate & pathways

Hydrolysis

Currently viewing:

Administrative data

Link to relevant study record(s)

Referenceopen allclose all

Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles (peer reviewed in OECD-SIDS)
Qualifier:
no guideline followed
Principles of method if other than guideline:
Experimental data
GLP compliance:
no
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
GC-MS
Transformation products:
not measured
pH:
7
Temp.:
25 °C
Hydrolysis rate constant:
ca. 0 min-1
DT50:
2.6 yr
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: r2 >=0.9

1.   The Arrhenius parameters for both the neutral and alkaline reactions are presented in the table below:

Table: Arrhenius parameters for ethyl chloride for the neutral and alkaline reactions

 

A (L mol-1 min-1)

E (kJ/mol)

K (25°C, pH 7)

t1/2**

Arrhenius parameters for the neutral reaction*

(1.4 ± 0.3) x 1013

110.8 ± 3.8

Kn= 5.1 x 10-7

2.6 years

Arrhenius parameters for the alkaline reaction**

 

(3.7 ± 0.8) x 1013

101.2 ± 10.2

Kb= 6.7 x 10-12

-

*  ‘‘Neutral’’ hydrolysis carried out in solutions of 0.01 M HCl 

** Alkaline hydrolysis measured in NaOH solutions ranging from 0.1 to 0.001 M NaOH. The alkaline hydrolysis half-life is determined by the “neutral” reaction.

 

Based on the results, a brominated compound is more reactive than its chlorinated analogue by a factor of 10 to 100, by both neutral and alkaline reaction mechanisms. Greater reactivity of Br versus Cl compounds is due to lower activation energy and entropy effects that also contribute in the same direction. Within a series of halogenated compounds, the order of reactivity is identical for brominated and chlorinated analogues. Substitution of F for Cl in the ethanes causes an enormous drop in neutral hydrolysis.
Conclusions:
Both the neutral and alkaline reaction rates for the hydrolysis of chloroethane were determined by Jeffers and Wolfe (1996). The neutral reaction measured in 0.01 M HCl at 25 °C was found to predominate, with a rate constant of 5.1 x 10-7, resulting in an estimated half-life for chloroethane of 2.6 years.
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles (peer reviewed in OECD-SIDS)
Qualifier:
no guideline followed
Principles of method if other than guideline:
Literature review
GLP compliance:
no
Radiolabelling:
no
Analytical monitoring:
no
Estimation method (if used):
Kinetic based calculations
Transformation products:
not specified
pH:
7
Temp.:
25 °C
Hydrolysis rate constant:
0 s-1
DT50:
38 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 38 d also at pH 4 and 9

1. Hydrolysis kinetic

Overall hydrolysis is a transformation of an organic chemical with water, resulting in the formation of a new covalent bond with OH. The overall reaction may be summarized as

R-X + H2O → R-OH + HX

X represents a good leaving group (an atom much more electronegative that carbon, e.g. Cl). The detailed mechanism may involve a protonated or anionic intermediate or a carbonium ion, or any combination of these intermediates. But whatever the mechanism, the rate law for hydrolysis of substrate RX usually can be put in the form

-d[RX]/dt = KH [RX] = KB [OH-][RX] + KA [H+][RX] + KN’ [H2O] [RX] (1)

Where KA, KB and KN are the second-order rate constants for acid and base catalysed and neutral processes, respectively. In water is the KN’ [H2O] a constant (KN). KH is the observed or estimated rate constant for hydrolysis at constant pH. Equation (1) assumes that the individual rate processes for the acid, base and neutral hydrolysis obey first-order kinetics with the hydrolysable chemical, RX, it is possible to write the following equation for KH:

KH = KB [OH-] + KA [H+] + KN (2)

From the equilibrium term for the ionisation of water, KW:

KW = [H+] [OH-] = 1 x 10 -14 (3)

It is possible to substitute for [OH-] in the equation (2), giving

KH = KB ∙ KW / [H+] + KA [H+] + KN (4)

Because KH is a pseudo first-order rate constant at a fixed pH, the half-life of hydrolysis can be calculated as below:

t1/2 = ln2 / KH = 0.693 / KH (5)

pH Dependence:

From the equation (5) it is evident how pH affects the overall rate constant for hydrolysis, KH: at high or low pH (high OH- or H+) either base or acid catalysed term is usually dominant, while at pH 7 the neutral processes can often be the most important. However, the detailed relationship of pH and rate depends on the specific values of KA, KB and KN.

The typical log KH vs. pH can be plotted to demonstrate for the chemicals which undergo acid, water and base catalysed hydrolysis at a specific pH with a curve composited of three straight lines with slops +1, 0 and -1 respectively as below:

(a) log KH = log (KB ∙ KW ) + pH

(b) log KH = log KN

(c) log KH = log KA – pH

Most log KH vs. pH curves are found to have one or two intercepts corresponding to pH values where two kinds of rate processes contribute equally to the overall process. Thus in the Figure 1 the intercept IAN corresponds to a pH value where KA [H+] = KN, similarly, IBN corresponds to KB [OH-] = KN. In cases where KA, KB or KN = 0, only one intercept is observed. Values of pH corresponding to the I may be calculated readily from the values of KA, KB and KN as below:

IAN = - log (KN / KA) (6)

INB = - log (KB ∙ KW / KN) (7)

IAB = - [log (KB ∙ KW / KN)] / 2 (8)

Use of intercepts in tabulating dat on rates of hydrolysis as a function of pH greatly simplifies the task of estimating the effect of pH on the rate constant KH for a specific compound.

2. Rate constants for Chloroethane

Chloraethane hydrolyses in water by neutral and base catalysed reaction to given ethanol and hydrogen chloride. The hydrochloric acid formed dissociates at the neutral pH of most natural waters and forms a chloride salt. No acid catalysed processes observed.

The rate constant of 1.x 10-4 by neutral and base catalysed reaction was reported for chloraethane at 100.2 °C. Rate constants of KN for hydrolysis reaction at 25 °C and pH was extrapolated based on the experimental rate constant and the temperature coefficient.

3. Hydrolysis as a function of pH for Chloroethane

The intercepts I, corresponding to pH values where two rate processes contribute equally to the observed rate constants, KN and KB for several primary alkyl halides are listed in the table below:

Organic halides KN s-1        KB [OH-] s-1 INB*

MeF              7.44 x 10-10       5.82 x 10-14 11.1

MeCl              2.37 x 10-8       6.18 x 10-13 11.6

MeBr              4.09 x 10-7       1.41 x 10-11 11.5

MeI               7.28 x 10-8       6.47 x 10-12 11.0

CH2CHCH2Cl   1.16 x 10-7    6.42 x 10-12 11.3

C6H5CH2Cl       1.28 x 10-5                   > 13

*These values were calculated from the equations (6), (7) and (8).

The intercepts INB lying above pH 11 indicate that neutral hydrolysis rate term for these compounds will dominate over typical environmental pH values of 4 to 9. Only above pH 11 will the base catalysed rate term contribute to the overall hydrolysis rate term. Likewise, hydrolysis of chloroethane is considerd not to be affected by pH values ranged from 4 to 9.

Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles (peer reviewed)
Qualifier:
no guideline followed
Principles of method if other than guideline:
Literature review
GLP compliance:
no
Radiolabelling:
no
Estimation method (if used):
Literature review
Transformation products:
not measured
pH:
5.6
Temp.:
25 °C
Hydrolysis rate constant:
0 min-1
DT50:
2.54 yr
Type:
(pseudo-)first order (= half-life)
pH:
7
Temp.:
25 °C
Hydrolysis rate constant:
0 min-1
DT50:
2.54 yr
Type:
(pseudo-)first order (= half-life)
pH:
5.6
Temp.:
10 °C
Hydrolysis rate constant:
0 min-1
DT50:
27.2 yr
pH:
7
Temp.:
10 °C
Hydrolysis rate constant:
0 min-1
DT50:
27.2 yr

Kinetic parameters for hydrolysis of chloroethane based on the data of Jeffers (1995) and Kollig (1990)

Arrhenius Parameters:

Kb: A= 3.72E+13 [L/(mol*min)]; EA=101200 (J/mol)

Kn: A= 1.37E+13 [L/(mol*min)], EA = 110800 (J/mol)

25 °C Data:

pH 7: Kobs = 5.18E-07; t1/2= 2.54 yr

pH 5.6: Kobs = 5.18E-07; t1/2= 2.54 yr

10 °C Data:

pH 7: Kobs = 4.84E-08; t1/2= 27.2 yr

pH 5.6: Kobs = 4.84E-08; t1/2= 27.2 yr

Dominat hydrolysis products are ethanol and HCl.

Description of key information

Chloroethane hydrolyses in water. The estimated half-life was reported between 38 d and 2.6 years at 25 °C, pH 4, 7 and 9.

Key value for chemical safety assessment

Additional information

Chloroethane (CAS 75-00-3) is not expected to be present in natural waters due to the high vapour pressure of the substance. Thus hydrolysis is not expected to be an important environmental fate process. If present in water, chloroethane is susceptible to slow chemical hydrolysis and forms ethanol and hydrochloric acid as reaction products. The hydrochloric acid formed dissociates at the neutral pH of most natural waters and forms a chloride salt. In water ß-elimination to given olefin and hydrogen halides is rare except at high temperature (ATSDR, 1998).

The hydrolytic half-life of chloroethane is not known with certainty. The hydrolytic half-life in water at 25 °C and pH 7 was estimated to be 38 days based on a reaction rate constant extrapolated from experimental data at 100 °C (Mabey and Mill 1978). However, both the neutral and alkaline reaction rates for the hydrolysis of chloroethane were determined by Jeffers and Wolfe (1996). The neutral reaction measured in 0.01 M hydrochloric acid at 25 °C was found to predominate, with a rate constant of 5.1 x 10-7 min-1, resulting in an estimated half-life for chloroethane of 2.6 years.

Chloroethane undergoes neutral and base catalysed hydrolysis to given alcohol. The estimated intercept INB for chloroethane is above pH 11, meaning base catalysed reactions are not important at pH is lower than 11. As no acid catalysed reaction observed, pH conditions in natural waters (pH 4 – 9) are not to have a significant effect on hydrolysis rate constant and half-life for chloroethane.

Reference

ATSDR (1998). Toxicological profiles for Chloroethane. U.S. Department of Health and human services, Public Health Service, Agency for Toxic Substances and Disease Registry, Division of Toxicology, Atlanta, GA